U.S. Pat. No. 3,496,401, of which one of the present inventors is the patentee, discloses the mechanism involved in the operation of tungsten-iodine incandescent lamps and provides glass compositions especially suitable as envelopes for such lamps. Thus, as is explained therein, molecular iodine is used in such lamps to inhibit blackening of the envelope and to improve bulb life by redepositing evaporated tungsten on the filament. During operation of the lamp, tungsten moves from the filament to the envelope wall by normal evaporation and diffusion. The molecular iodine placed within the lamp is converted by pyrolysis, in the close proximity of the filament, to iodine atoms which diffuse to the wall of the envelope and there react with adsorbed tungsten to form tungsten iodide which, being volatile, diffuses to the filament. The tungsten iodide is decomposed at the filament, leading to the deposition of tungsten on the filament and the production of iodine atoms, which then again migrate to the envelope wall to begin the cycle anew. As can be recognized, the mechanism and conditions for the regenerative action of the tungsten-iodine cycle is founded in the iodine-molecular dissociation and the formation and decomposition of tungsten iodide.
Initial attempts to employ gaseous bromine and chlorine in place of iodine were commercially unsuccessful because of the increased reactivity of those halogens when compared with iodine. The tungsten supports and leads were rapidly attacked such that lamp life was severely reduced. However, because (1) it is easier to keep the bromine cycle going than the iodine cycle (larger concentration range), (2) the light emitted by the bromine lamp is whiter than that of the iodine lamp, and (3) the inherent economics that could be enjoyed employing bromine, further investigation was undertaken which demonstrated that gaseous hydrogen bromine (HBr), resulting from the decomposition of a bromine-containing hydrocarbon, was thermally stable at the temperatures seen by the molybdenum or tungsten supports and leads and, therefore, protected those elements against bromide attack. This has resulted in making tungsten-bromine lamps, operating in a manner generally similar to tungsten-iodine lamps, now practical.
As was observed in U.S. Pat. No. 3,496,401, supra, envelopes or bulbs for the tungsten-iodine lamps were made of fused quartz or 96% silica compositions sold under the trademark VYCOR because of their ability to withstand the very high operating temperatures encountered during lamp operation. Thus, glasses are required which have strain points in excess of 700.degree.C. to avoid the deformation at operating temperatures. (The strain point of a glass is considered to be the temperature at which the internal stress is substantially relieved and corresponds to a viscosity of 10.sup.14.5 poises when measured in accordance with ASTM Method Designation C336.) Nevertheless, fused quartz and 96% silica glasses are difficult to lamp work and have such low coefficients of thermal expansion that expensive molybdenum-foil seals must be utilized to introduce the lead wires into the lamps.
The alkaline earth aluminosilicate glasses disclosed in U.S. Pat. No. 3,496,401, supra, were specifically designed for use in the tungsten-iodine lamps. Such glasses consisted essentially, by weight, of 10-25% of an alkaline earth metal oxide, 13-25% Al.sub.2 O.sub.3, 55-70% SiO.sub.2, O-10% B.sub.2 O.sub.3, and less than about 0.1% of alkali metal oxide. The patentee had noted that the presence of more than about 0.1% alkali metal oxide resulted in a white coating being developed on the inner surface of the lamp envelope. While such glasses were, indeed, superior in lamp working properties to fused quartz and 96% silica glasses and their coefficients of thermal expansion considerably higher so as to be more compatible with tungsten, the melting temperatures demanded for melting such glasses exceeded the capabilities of conventional melting units and necessarily led to severe attack of the melting unit refractories.